1. Field of the Invention
The present invention relates to an optical grating coder; and, more particularly, to a chirped optical fiber grating coder implementing predetermined codewords for use in an OCDMA (Optical Code Division Multiple Access) system, a fabricating method and an apparatus therefor.
2. Description of the Related Art
An optical fiber grating is generally fabricated by using the refractive index variation of a core of an optical fiber when a core doped with Germanium (Ge) is exposed to an ultraviolet light. A short-period optical fiber grating is typically referred to an optical fiber grating. The short-period optical fiber grating has a grating period of 0.5 xcexcm, approximately, and a length less than 10 mm. The short-period optical fiber grating may be used as a reflection filter for filtering light wavelengths, external reflection mirrors of an optical fiber laser and a semiconductor laser, an optical fiber grating Fabry-Perot etalon, and the like.
A fabricating method of an optical fiber grating is generally classified into two: one is to form an interference fringe induced by the interference of ultraviolet lights and to imprint the interference fringe into a core of an optical fiber; and the other, which is gaining popularity, is to utilize a phase mask creating an interference pattern of the ultraviolet lights in the fabrication process of the optical fiber grating.
FIG. 1 shows an apparatus for fabricating a short-period optical fiber grating by utilizing a phase mask according to the prior art. The apparatus 100 of FIG. 1 comprises a light source 101, a convex lens 102, a phase mask 104, and a single-mode optical fiber 106 that is typically used in a telecommunication system. In FIG. 1, in order to measure the transmission characteristics of a short-period fiber grating 112 imprinted into the fiber 106, an optical spectrum analyzer 106 and an optical amplifier 110 are also provided in the apparatus 100.
The light source 101 may be one of a KrF excimer laser (248 nm), a second harmonic laser (244 nm) of an Ar laser, and a fourth harmonic laser (265 nm) of an Nd:YAG laser. The light source 101 irradiates a coherent ultraviolet light beam to the fiber 106. The ultraviolet light beam reaches on the fiber 106 through the convex lens 102 and the phase mask 104. In practice, after the diffraction by the phase mask 104, a zero-order beam has been suppressed to less than 5% of incident light beams. Light beams exiting the phase mask 104 are the diverging plus-one and minus-one orders, each of which contained typically more than 30% of the incident light beams. The diffracted light of the plus-one and minus-one orders form interference fringes prior to reaching on the fiber 106. The interference fringes are imprinted into the fiber 106 to produce the short-period fiber grating 112.
A Ge-doped or a Ge- and B(boron)-doped optical fiber is commonly used in the fabrication of the short-period fiber grating 112. The fiber 106 may be the Ge-doped or the Ge- and B-doped optical fiber. As is well known in the art, it is required to perform a hydrogen (H2) treatment on the fiber 106 in order to form the short-period fiber grating 112 thereon.
FIG. 2 presents an apparatus for fabricating a chirped fiber grating according to the prior art. The apparatus 200 of FIG. 2 comprises a light source 201, a concave lens 202, a convex lens 204, a phase mask 206, a single-mode optical fiber 208 for use in the telecommunication system. Similar to the optical amplifier 110 and the optical spectrum analyzer 108 of FIG. 1, an optical amplifier 210 and an optical spectrum analyzer 212 are also provided for measuring the transmission characteristics of a chirped fiber grating 214.
As seen from FIG. 2, a grating period of the chirped fiber grating 214 is gradually changed along the axis of the fiber 208 different to that of the short-period fiber grating 112 shown in FIG. 1. The chirped fiber grating 214 induces a time difference to light beams irradiated from the optical amplifier 210 to the fiber 208 depending on the wavelengths of the light beams. Using the time difference induced by the chirped fiber grating 214, it is possible to compensate the dispersion of the light beams from the optical amplifier 210 propagating the fiber 208 depending on the dispersion characteristics thereof, so that the chirped fiber grating 214 may be suitable for an ultrahigh-speed optical communication system.
In order to use the chirped fiber grating 214 as an optical grating coder in an OCDMA (Optical Code Division Multiple Access) system, it is required to connect to each other short-period fiber gratings (or chirped fiber gratings), each of which has a different grating period, or to fabricate a super-structural fiber grating having a length enough for implementing desired codewords.
Specifically, after forming one short-period fiber grating on an optical fiber, by moving the optical fiber to a predetermined direction in order to prevent from overlapping the short-period fiber grating formed on the optical fiber with another short-period fiber grating to be formed thereon, a chirped optical fiber grating coder consist of a plurality of the short-period gratings having different grating periods is fabricated enough for implementing desired codewords. To use the process described above, however, would call for an additional procedure of alternating a plurality of phase masks during the fabrication of the chirped optical fiber grating coder. Further, the length of the chirped optical fiber grating coder fabricated would be elongated as a number of the short-period fiber gratings for implementing the desired codewords are increased.
Consequently, there are some drawbacks that the conventional fabrication process of the chirped optical fiber grating coder takes much time and a plurality of phase masks is necessary.
It is, therefore, an objective of the present invention to provide a chirped optical fiber grating coder implementing predetermined codewords for use in an OCDMA system, a fabricating method and an apparatus therefor.
In accordance with one aspect of the present invention, there is provided an optical fiber grating coder comprising predetermined codewords imprinted into an optical fiber in the form of a plurality of striations through an apparatus including a light source, an amplitude mask, and a phase mask.
In accordance with another aspect of the present invention, there is provided a method for fabricating an optical fiber grating coder, comprising the steps of: a) providing a light source; b) providing an amplitude mask designed for predetermined codewords; c) providing a phase mask; d) providing an optical fiber; e) exposing the optical fiber to a light beam irradiated from the light source passing sequentially through the amplitude and phase masks; and f) forming an optical fiber grating coder on the optical fiber, including the predetermined codewords in the form of a plurality of stripes.
In accordance with still another aspect of the present invention, there is provided an apparatus for fabricating an optical fiber grating coder, comprising: a light source; an amplitude mask designed for predetermined codewords; a phase mask; and an optical fiber.